Rotational movement of piston rings is an important, yet poorly understood, parameter in engine operation and durability. Gap position of the top compression ring has been shown to affect hydrocarbon exhaust emissions, and alignment of ring gaps is thought to cause an increase in oil consumption. The absence of piston-ring rotational movement in diesel engines can cause localized wear and carbon deposits, while excessive rotational movement can contribute to high wear on the ring sides.
Ring rotation is driven only by transient forces which can not readily be taken into account in a new or modified engine design. Accordingly, it is advantageous to be able to measure ring rotation while the engine is operating in order to evaluate ring performance and diagnose the cause of rotational movement outside an acceptable range. In an SAE paper by the Research Laboratories of the General Motors Corporation, entitled "A Method for Measurement of Piston Ring Rotation", Schneider et al. suggest tracking ring rotation with a portable germanium detector. In order to overcome the inability of existing methods, having two Geiger counters/one source or one detector/two identical sources, to determine direction of rotation, the authors inserted in the piston ring a radioactive cobalt wire near the end gap, and a zinc wire at approximately a one hundred twenty degree spacing. In addition to difficulties with handling and decay of radioactive materials, the system is able to monitor only a single piston ring per cylinder.
U.S. Pat. No. 4,143,319 to Rouam teaches using an eddy current detector in the cylinder wall to monitor the degree of wear for a wear-resistant piston ring coating, such as chromium. A decreased thickness of the low permeability chromium layer causes the eddy currents generated in the high permeability steel ring to exceed a predetermined threshold. The patent further suggests that a deformation of the piston ring can be detected by a lack of coincidence in the measurements produced by two detectors mounted in the cylinder in diametric opposition.
U.S. Pat. No. 5,258,930 to Fukuyoshi et al. discloses a similar system but compares the induction detected in the coated top ring to that in the lower non-coated rings. The patent further suggests in alternative embodiments that the portion of the ring which is worn can be determined by varying the location of a coated notch in the ring around the circumference.
While the foregoing eddy current detector arrangements may provide some information about end gap position, the effect of signal noise and thermal or elastic calibration slippage make small variations in the detector output an unreliable indicator of position.
Despite the foregoing difficulties, eddy-current systems are generally immune to environmental contaminants such as oil, water dirt and dust prevalent with internal combustion engines. Accordingly, it is an object of the present invention to provide safe, reliable measurement of piston ring rotation by use of inductive detectors.
It is another object to permit simultaneous rotation measurement of multiple piston rings within a cylinder to permit determination of whether the end gaps are in alignment.
It is still another object of the invention to measure piston ring rotation using conventional piston rings. This helps ensure similar results will be achieved in unmodified rings used in production engines.